experience-mine: link at creation time, remove # from new keys

Update the experience mining prompt to output links alongside journal
entries. The LLM now returns a "links" array per entry pointing to
existing semantic nodes. Rust code creates the links immediately after
node creation — new nodes arrive pre-connected instead of orphaned.

Also: remove # from all key generation paths (experience miner,
digest section keys, observed transcript keys). New nodes get clean
dash-separated keys.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

poc-memory/src/agents/digest.rs

@@ -365,7 +365,7 @@ fn normalize_link_key(raw: &str) -> String {
     } else if key.contains('#') {
         let (file, section) = key.split_once('#').unwrap();
         if let Some(bare) = file.strip_suffix(".md") {
-            key = format!("{}#{}", bare, section);
+            key = format!("{}-{}", bare, section);
         }
     }